1. Field of the Invention
The invention relates to an apparatus for the uphill low pressure casting of molten metal in sand moulds conveyed in a synchronized manner with a casting station located on the conveyor, a molten metal-containing gas pressure holding furnace located at the casting station and having an uphill casting tube connectable to the sand moulds and a device for refilling the holding furnace with molten metal.
2. Description of the Prior Art
Uphill low pressure casting is widely used, particularly for light metals, preferably aluminium. The reason for this is that light metal melts very rapidly oxidize in air and undesired oxide skin is formed, which significantly impairs the quality of the casting. Thus, closed casting paths must be ensured. Where air contact is unavoidable, namely in the mould, turbulent movements of the melt must be avoided. This only takes place in a satisfactory manner using the uphill casting principle, in which the melt is forced out of a holding furnace under gas pressure into the mould. This casting principle is particularly advantageous if, as is increasingly the case, sand moulds are used, because as a result of the non-turbulent or only slightly turbulent melt flow the detachment of sand particles in the mould is avoided, which would give rise to quality losses on the finished casting.
Casting in sand moulds known from iron and steel casting foundries is nowadays possible with high cycle times because, as opposed to die casting, the sand moulds only have to be stopped at the casting station for the time necessary for filling the mould, whilst the overall cooling process and the solidification of the casting can take place on a following cooling section. There are also limits with respect to die casting with increasing casting weight. Conversely with low pressure casting problems arise in that the conventional gas pressure holding furnaces, which can only contain a specific melt quantity, fail with high casting capacities of several tonnes per hour and even with lower casting capacities, but high cycle times for the refilling of the melt require too much time. During refilling the mould conveying must also be stopped.
In the case of downhill low pressure casting the refilling of the melt container takes place more or less continuously by means of an upstream melting pot with a dip pipe introduced into the holding furnace. This would lead in the case of uphill casting, in which the mould to be filled is above the holding furnace, to a geodesic height of the pot, which would be associated with a considerable height of fall of the melt. Thus, once again there is an increase in oxide formation risks and the gas quantity absorbed by the melt rises, which would in turn significantly impair the casting quality.
The invention provides an apparatus for uphill low pressure casting, which on the one hand makes it possible to maintain high cycle times in the conveyor of the sand moulds and on the other permits a high casting capacity (To/h).
On the basis of the aforementioned apparatus, this problems of the prior art are solved in that the device for refilling has at least one pressure-tight melt container with a melt supply and a bottom outlet and a filling tube located in the holding furnace with a seal positioned outside the same, that the melt container by means of its outlet can be coupled in pressure-tight manner to the holding furnace filling tube and that the holding furnace is connected to the melt container by means of a pneumatic pressure compensating line.
According to the invention on the holding furnace is placed a refilling container, which contains a melt supply and by means of a fill tube dipping into the holding furnace transfers the melt to the holding furnace. The transfer of the melt from the refilling container into the holding furnace is possible without any additional lowering of the gas pressure in the holding furnace in that the latter is connected to the refilling container by means of a pneumatic pressure compensating line, which link the gas cushion in the holding furnace and in the refilling container. Thus, for the same reason, the coupling between the outlet of the melt container and the holding furnace filling tube is pressure-tight.
The sealing of the filling tube is preferably controllable as a function of the conveying cycle of the sand moulds.
The control can take place in such a way that following each casting cycle the seal is opened for sufficiently long to permit the refilling of the melt volume necessary for filling the sand mould. However, the refilling cycles can also be longer and then with each refilling cycle a larger melt volume can be transferred into the holding furnace. With this operation in a pressure-compensated state, the filling dynamics during the refilling of the melt is limited to the metallostatic pressure difference, because the gas pressure in the furnace changing during the casting cycle by means of the pressure compensating line being always also present at the melt level in the refilling container, so that the gas pressure in the holding furnace is completely compensated. Unlike in the case of filling by means of pots with the same geodesic height, in the case of the apparatus constructed according to the invention the melt is transferred into the furnace in a closed system and then the melt can continuously overflow without any air suction. The flow rate in the filling tube can be reduced to the desired level by adopting known measures.
Advantageously the filling tube ends close to the bottom of the holding furnace, so that also in the tatter there are no turbulent melt movements during the refilling cycle.
In another advantageous development the filling tube is cardan-mounted in the holding furnace in order to facilitate the coupling of the bottom outlet of the melt container to the furnace.
A preferred variant of the invention uses the melt container constructed as a transportation container with a seal at the bottom outlet and is replaceable by a full transportation container after emptying.
The seals at the bottom outlet and at the filling tube must be closed for replacement purposes. The pressure compensating line must also be detached from the container and the latter is then decoupled and replaced by a full melt container. The latter is coupled in the vicinity of the seals to the filling tube, the pressure compensating line is connected and operation can then commence. Replacement can also take place during casting, so that the sand moulds can still be cast with the given time cycle.
It is advantageous if, prior to the coupling to the filling tube and the connection of the pressure compensating line, the transportation container is pretensioned with gas pressure, so that a pressure drop in the holding furnace is avoided on coupling and it is possible to start the refilling from the melt container immediately after coupling.
The transportation container is preferably placed in a transportation support by means of which it is insertable in receptacles on the holding furnace.
This ensures a simple, rapid positioning of the refilling container in the coupling position on the holding furnace, so that limited time is needed for the replacement process and in conjunction with the aforementioned features any air access to the melt system is prevented. Alignment errors in the coupling position can be easily compensated by the cardan mounting of the filling tube on the holding furnace.
For high casting capacities of up to several tonnes per hour, in general a larger stationary melt supply will be kept available, e.g. in a fore-hearth or in a treatment furnace, because with such casting capacities the replacement of the refilling container can no longer take place sufficiently quickly or is complicated and costly as a result of the then necessary frequent replacement. According to the invention, for such a case the melt container is placed as a lock container between the holding furnace and the stationary melting plant or stationary melt supply and can be refilled by means of a dip pipe with a front side seal outside the melt container.
Refilling can take place by means of an open pot coupled by means of the seal to the dip pipe, so that once again the lock container and holding furnace can be operated as a pressure-tight system with overpressure. During refilling cycles the lock container is briefly pressure-relieved and following the transfer of the melt into the lock container the latter is placed under pressure again.